It will be better if you set this at server side like following.
HTML
<asp:hyperlink id="HyperLink1" runat="server" NavigateUrl="~/Default2.aspx?id=" >MyLink</asp:hyperlink>
Code Behind
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
HyperLink1.NavigateUrl += (3 + 6).ToString();
}
}